Radiation source, lithographic apparatus, device manufacturing method, and device manufactured thereby

ABSTRACT

A radiation source includes an anode and a cathode for creating a discharge in a vapor in a space between anode and cathode and to form a plasma of a working vapor so as to generate electromagnetic radiation. The cathode defines a hollow cavity in communication with the discharge region through an aperture that has a substantially annular configuration around a central axis of said radiation source so as to initiate said discharge. A driver vapor is supplied to the cathode cavity and the working vapor is supplied in a region around the central axis in between anode and cathode.

This is a continuation application of U.S. application Ser. No.09/893,347, filed Jun. 28, 2001, now U.S. Pat. No. 6,667,484 whichclaims priority from European Patent Application No. 00202304.2, filedJul. 3, 2000, the entire contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to lithographic projectionapparatus and specifically to a plasma discharge radiation source foruse therein.

2. Description of the Related Art

The term “patterning structure” as here employed should be broadlyinterpreted as referring to structure that can be used to endow anincoming radiation beam with a patterned cross-section, corresponding toa pattern that is to be created in a target portion of the substrate;the term “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such patterning structure include:

A mask. The concept of a mask is well known in lithography, and itincludes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired;

A programmable mirror array. An example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, theundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-addressablesurface. The required matrix addressing can be performed using suitableelectronic means. More information on such mirror arrays can be gleaned,for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which areincorporated herein by reference. In the case of a programmable mirrorarray, the support structure may be embodied as a frame or table, forexample, which may be fixed or movable as required; and

A programmable LCD array. An example of such a construction is given inU.S. Pat. No. 5,229,872, which is incorporated herein by reference. Asabove, the support structure in this case may be embodied as a frame ortable, for example, which may be fixed or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning structure ashereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningstructure may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (silicon wafer) thathas been coated with a layer of radiation-sensitive material (resist).In general, a single wafer will contain a whole network of adjacenttarget portions that are successively irradiated via the projectionsystem, one at a time. In current apparatus, employing patterning by amask on a mask table, a distinction can be made between two differenttypes of machine. In one type of lithographic projection apparatus, eachtarget portion is irradiated by exposing the entire mask pattern ontothe target portion at once; such an apparatus is commonly referred to asa wafer stepper. In an alternative apparatus—commonly referred to as astep-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed V at which thesubstrate table is scanned will be a factor M times that at which themask table is scanned. More information with regard to lithographicdevices as here described can be gleaned, for example, from U.S. Pat.No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (resist).Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Twin stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO98/40791, incorporated herein by reference.

In a lithographic apparatus the size of features that can be imaged ontothe wafer is limited by the wavelength of the projection radiation. Toproduce integrated circuits with a higher density of devices, and hencehigher operating speeds, it is desirable to be able to image smallerfeatures. While most current lithographic projection apparatus employultraviolet light generated by mercury lamps or excimer lasers, it hasbeen proposed to use shorter wavelength radiation of around 13 nm. Suchradiation is termed extreme ultraviolet, also referred to as XUV or EUV,radiation. The abbreviation ‘XUV’ generally refers to the wavelengthrange from several tenths of nm to several tens of nanometers, combiningthe soft x-ray and Vacuum UV range, whereas the term ‘EUV’ is normallyused in conjunction with lithography (EUVL) and refers to a radiationband from approximately 5 to 20 nm, i.e. part of the XUV range.

A radiation source for XUV radiation may be a discharge plasma radiationsource, in which a plasma is generated by a discharge in a vapor betweenan anode and a cathode and in which a high temperature discharge plasmamay be created by Ohmic (resistive) heating by a (pulsed) currentflowing through the plasma. As used herein, “vapor” is intended broadlyto include a gas, a suspension or a mixture. Further, compression of aplasma having some volume due to a magnetic field generated by a currentflowing through the plasma may be used to create a high temperature,high density plasma on a discharge axis (dynamical pinch effect).Kinetic energy of the pinching plasma is directly transferred to theplasma temperature and hence to short-wavelength radiation. A dynamicalpinch would allow for a discharge plasma having a considerably highertemperature and density on the discharge axis, offering an extremelylarge conversion efficiency of stored electrical energy into thermalplasma energy and thus into XUV radiation.

It has been proposed by R. Lebert, K. Bergmann, G. Schriever and W. Neffin a presentation entitled ‘A gas discharge based radiation source forEUVL’, Sematech Workshop Monterey (1999), to employ a hollow cathode fortriggering plasma creation. A very effective way of self-initiation of adischarge may be obtained by a so-called transient hollow cathodedischarge (THCD) in the hollow cathode. The radiation source as proposedby Lebert et al. is an axi-symmetric system with a specially configuredcathode having a small aperture on-axis with a large cavity behind itforming a hollow cathode region. However, the hollow cathode generatesthe discharge breakdown and thus a plasma only on or in a small volumearound the discharge axis, which does not allow making (full) use of thedynamical pinch effect referred to above. Further, the volume taken by aplasma around the discharge or central axis is generally badlycontrollable and its stagnation on the discharge axis after compressionis therefore not sufficiently predictable to have an exactly timed pulseof generated XUV radiation.

Another drawback of a plasma discharge radiation source having theconventional central hollow cathode is that the plasma created may erodeand change the form of the aperture of the hollow cathode, since it ispresent on the axis on which the high temperature plasma having aconsiderable density is created. The aperture will therefore be damagedby unavoidable axially oriented plasma jets, which limits the lifetimeof the cathode and decreases the maintenance interval of the radiationsource. Further, proper functioning of the hollow cathode for triggeringthe plasma is dependent on a predetermined relation between size of theaperture and depth of the cavity. Erosion of the aperture thereforeundesirably influences the triggering instant of the plasma and thetiming of the pulse of generated XUV radiation.

SUMMARY OF THE INVENTION

In one aspect of the present invention a radiation source is providedhaving an effective self-initiation that allows making use of thedynamical pinch effect to create a high-temperature, high-density plasmafor an enhanced conversion efficiency of electrical energy intoradiation.

Another aspect the invention provides a radiation source having a longmaintenance interval.

Another aspect of the invention provides a radiation source having awell-defined timing of generated pulses, or shots, of XUV radiation.

According to one aspect of the present invention there is provided aradiation source comprising an anode and a cathode that are configuredand arranged to create a discharge in a vapor in a space between saidanode and cathode and to form a plasma of a working vapor so as togenerate electromagnetic radiation. The cathode further defines a hollowcavity in communication with the discharge region through an aperturethat has a substantially annular configuration around a central axis ofsaid radiation source so as to initiate said discharge. The workingvapor may include, for example, xenon, lithium vapor or tin vapor.

The present invention can provide that the discharge is being initiatedby the annular hollow cathode at a predetermined distance from a centralaxis of the radiation source and take advantage of the dynamical pincheffect. The discharge is created at least at a distance from the centralaxis corresponding to the annular aperture so as to create an initialplasma. An electrical current flowing through the plasma between anodeand cathode generates a magnetic field compressing the plasma from thedistance (or radius) corresponding to the annular aperture towards thecentral axis so as to create a dense and hot plasma.

The discharge plasma created above the annular aperture may be chosen tohave a low density such as not to cause erosion of the aperture. Boththe density of the plasma and its distance to the annular aperture willincrease upon compression towards the central axis. The distance ofannular aperture may be chosen large enough not to cause erosion of theaperture at the final stagnation, or collapse, of the plasma on thecentral axis.

Further, the predetermined distance between annular hollow cathodeprovides a control radius where plasma compression starts, which resultsin a well-controllable timing of its collapse and generation of a pulseof XUV radiation.

In certain embodiments a driver vapor is supplied to said cavity.Further, the working vapor may be conveniently supplied in a regionaround said central axis of said space between said anode and cathode.In such embodiments control over the generation of pulses of XUVradiation is improved.

The hollow cathode provides for an effective self-organized dischargeinitiation having the practical feature that high repetition rates alongwith a high reproducibility may be achieved. The operation can basicallybe auto-triggered. To further increase the exact timing of the dischargeand finally of the generation of a pulse of XUV radiation, oneembodiment of the radiation source according to the invention includes atrigger electrode that is inserted in the cavity. When the radiationsource is in a state that auto-triggering is almost going to take place,a voltage pulse applied to the trigger electrode will cause such adisturbance of the electrical field inside the cavity that the processwill trigger. Any uncertainty in the timing of auto-triggering will inthis way be eliminated by exactly timing a trigger pulse being appliedto the trigger electrode. Appropriate electrical circuitry is employedto apply a voltage pulse to said trigger electrode.

To provide for a stable shot-to-shot electrically stored energy and avery fast supply of electrical energy to anode and cathode, theradiation source may include a capacitor connected to said anode andcathode, and may further include a charging circuit connected to saidcapacitor, said charging circuit including a further capacitor and atransformer for electrically coupling said capacitor and said furthercapacitor. Switches may control charging of said capacitor by saidfurther capacitor, and of said further capacitor by a source ofelectrical power.

An electrical insulator will be provided to separate anode and cathode.To prevent degradation of the insulator by lithium vapor or tin vapor, apath between a region of said space between said anode and cathode wheresaid vapor is provided and said electrical insulator is constructed andarranged to define a space for said vapor to condense along said path.

According to a further aspect of the present invention there is provideda lithographic projection apparatus including a radiation system forproviding a projection beam of radiation, a support structure forsupporting patterning structure, the patterning structure serving topattern the projection beam according to a desired pattern, a substratetable for holding a substrate; and a projection system for projectingthe patterned beam onto a target portion of the substrate, wherein saidradiation system includes a radiation source as described above.

The present invention also provides a device manufacturing methodincluding providing a substrate that is at least partially covered by alayer of radiation-sensitive material, providing a projection beam ofradiation using a radiation system comprising a radiation source asdescribed above, using patterning structure to endow the projection beamwith a pattern in its cross-section, and projecting the patterned beamof radiation onto a target portion of the layer of radiation-sensitivematerial.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andextreme ultraviolet radiation (XUV or EUV, e.g. having a wavelength inthe range of 5 to 20 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention and its attendant advantages will be describedbelow with reference to exemplary embodiments and the accompanyingschematic drawings, in which like parts are indicated by likereferences, and in which:

FIG. 1 depicts a lithographic projection apparatus comprising aradiation source according to the invention;

FIGS. 2a to 2 e depict a radiation source according to a firstembodiment of the invention and various stages of discharge initiation,plasma creation and plasma compression;

FIG. 3 depicts a radiation source according the a second embodiment ofthe invention;

FIG. 4 depicts a radiation source according to a third embodiment of theinvention;

FIG. 5 depicts an electrical scheme for charging a capacitor bank andfor supplying a trigger pulse to a trigger electrode of a radiationsource according to a fourth embodiment of the invention;

FIG. 6 depicts a radiation source according to yet another embodiment ofthe invention; and

FIG. 7 depicts a detail of an alternative embodiment of a part of theradiation source of FIG. 6.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENITON

Embodiment 1

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatusincludes a radiation system Ex, IL, for supplying a projection beam PBof XUV radiation. In this particular case, the radiation system alsocomprises a radiation source LA, a first object table (mask table) MTprovided with a mask holder for holding a mask MA (e.g. a reticle), andconnected to first positioning means for accurately positioning the maskwith respect to item PL, a second object table (substrate table) WTprovided with a substrate holder for holding a substrate W (e.g. aresist-coated silicon wafer), and connected to second positioning meansfor accurately positioning the substrate with respect to item PL, and aprojection system (“lens”) PL for imaging an irradiated portion of themask MA onto a target portion C (e.g. comprising one or more dies) ofthe substrate W.

As here depicted, the apparatus is of a reflective type (i.e. has areflective mask). However, in general, it may also be of a transmissivetype, for example (with a transmissive mask). Alternatively, theapparatus may employ another kind of patterning structure, such as aprogrammable mirror array of a type as referred to above. Since nomaterials are known to date that will transmit EUV (XUV) radiation, thelens will generally also consist of reflective elements. However, oneskilled in the art will appreciate that should lens elements whichtransmit in the EUV spectrum be developed, such a transmissive lenssystem could be employed within the scope of the present invention.

The source LA produces a beam of radiation. This beam is fed into anillumination system (illuminator) IL, either directly or after havingtraversed conditioning means, such as a beam expander, for example. Theilluminator IL may be adjustable for setting the outer and/or innerradial extent (commonly referred to as σ-outer and σ-inner,respectively) of the intensity distribution in the beam. Likewise, theilluminator IL may include the capability of producing a multipoleprofile, such as a quadrupole. In addition, it will generally comprisevarious other components, such as an integrator and a condenser. In thisway, the beam PB impinging on the mask MA has a desired intensitydistribution in its cross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus, but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors). The current invention and claimsencompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having been selectively reflected by the mask MA, the beam PBpasses through the lens PL, which focuses the beam PB onto a targetportion C of the substrate W. With the aid of the substrate tableactuators (and interferometric measuring apparatus IF), the substratetable WT can be moved accurately, e.g. so as to position differenttarget portions C in the path of the beam PB. Similarly, the mask tableactuators can be used to accurately position the mask MA with respect tothe path of the beam PB, e.g. after mechanical retrieval of the mask MAfrom a mask library, or during a scan. In general, movement of theobject tables MT, WT will be realized with the aid of a long-strokemodule (coarse positioning) and a short-stroke module (finepositioning), which are not explicitly depicted in FIG. 1. However, inthe case of a wafer stepper (as opposed to a step-and-scan apparatus)the mask table MT may just be connected to a short stroke actuator, ormay be fixed.

The depicted apparatus can be used in two different modes:

In step mode, the mask table MT is kept essentially stationary, and anentire mask image is projected at once (i.e. a single “flash”) onto atarget portion C. The substrate table WT is then shifted in the x and/ory directions so that a different target portion C can be irradiated bythe beam PB;

In scan mode, essentially the same scenario applies, except that a giventarget portion C is not exposed in a single “flash”. Instead, the masktable MT is movable in a given direction (the so-called “scandirection”, e.g. the y direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the lens PL (typically,M=¼ or ⅕). In this manner, a relatively large target portion C can beexposed, without having to compromise on resolution.

FIGS. 2a to 2 e show a discharge plasma radiation source according tothe invention. The source has cylindrical symmetry and comprises ananode 10 and a cathode 20 connected by an electrically insulatingcylindrical wall 30. An aperture 11 is provided in anode 10 on centralaxis A for passing electromagnetic radiation from the source. Cathode 20is provided with an annular aperture 21 around central axis A, and isfurther provided with a large cavity 22 behind aperture 21. Cavity 22also has an annular configuration around central axis A, and the wallsof the cavity are a part of cathode 20. Appropriate electrical circuitry(not shown in FIGS. 2a-e) is connected to anode 10 and cathode 20 toprovide for a pulsed voltage V across the anode-cathode gap inside theradiation source. Further, a suitable working vapor, such as xenon orlithium vapor, at a certain pressure p is provided in between anode andcathode.

A discharge may take place at low initial pressure (p<0.5 Torr) and highvoltage (V<10 kV) conditions, for which the electron mean free path islarge compared to the dimension of the anode-cathode gap, so thatTownsend ionization is ineffective. Those conditions are characterizedby a large electrical field strength over vapor density ratio, E/N. Thisstage shows rather equally spaced equipotential lines EP having a fixedpotential difference, as is depicted in FIG. 2a.

The ionization growth is initially dominated by events inside the hollowcathode that operates at considerable lower E/N, resulting in a smallermean free path for the electrons. Electrons e from hollow cathode 20,and derived from a driver vapor within cavity 22, are injected into theanode-cathode gap, a virtual anode being created with ongoingionization, which virtual anode propagates from anode 10 towards hollowcathode 20, bringing the full anode potential close to the cathode, asis shown in FIG. 2b by unevenly distributed equipotential lines EP. Theelectric field inside the hollow cavity 22 of cathode 20 is nowsignificantly enhanced.

In the next phase, the ionization continues, leading to a rapiddevelopment of a high density plasma region inside the hollow cathode,immediately behind the cathode aperture 21. Finally, injection of anintense beam of electrons from this region into the anode-cathode gap,also shown in FIG. 2b, forms the final breakdown channel. Theconfiguration provides for a uniform pre-ionization and breakdown in thedischarge volume.

FIG. 2c shows that a discharge has been initiated and a plasma of theworking vapor has been created in the anode-cathode gap. An electricalcurrent will be flowing within the plasma from cathode 20 to anode 10,which current will induce an azimuthal magnetic field, having magneticfield strength H, within the radiation source. The azimuthal magneticfield causes the plasma to detach from the cylindrical wall 30 and tocompress, as is schematically depicted in FIG. 2c.

Dynamic compression of the plasma will take place, as further depictedin FIG. 2d, because the pressure of the azimuthal magnetic field is muchlarger than the thermal plasma pressure: H²/8π>>nkT, in which nrepresents plasma particle density, k the Boltzmann constant and T theabsolute temperature of the plasma. Electrical energy stored in acapacitor bank (not shown in FIG. 2) connected to anode 10 and cathode20 will most efficiently be converted into energy of the kineticimplosion during the full time of the plasma compression. Ahomogeneously filled pinch with a high spatial stability is created.

At the final stage of plasma compression, i.e. plasma stagnation oncentral, or discharge, axis A, the kinetic energy of the plasma is fullyconverted into thermal energy of the plasma and finally intoelectromagnetic radiation having a very large contribution in the XUVand EUV ranges.

The radiation source according to the invention may be operated at a lowinitial pressure of the working gas in the anode-cathode gap to providean easy interface of the source with a vacuum environment and to providea better pre-ionization in the discharge volume. Further, a low initialpressure and the hollow cathode concept allow a direct electricalcoupling of a capacitor bank with the anode-cathode gap. A low initialpressure is compensated by a reasonable radius of the annular hollowcathode to yield a dense resulting pinched plasma on discharge axis A.The resulting density is important since the total radiated power scaleswith the density squared. By choosing a larger distance between annularhollow cathode and discharge axis, a pinched plasma with a higherdensity is obtained.

The processes described with regard to FIGS. 2a to 2 e also apply to theembodiments of the invention which are disclosed below.

Embodiment 2

FIG. 3 shows a second embodiment of a radiation source according to theinvention. It shows the configuration of anode 10 and cathode 20, whichare kept separated by an electrical insulator 30 and which are connectedto a capacitor bank 40. A central part of the radiation source hascylindrical symmetry around central axis A. FIG. 3 further shows annularcathode aperture 21 and annular cathode cavity 22 around central axis A.

A driver vapor is supplied to cavity 22 via an inlet 25 so as to providea low pressure within the cavity. In the present embodiment, argon (Ar)is taken as the driver gas, but basically any gas, such as for instancehelium (He), neon (Ne) and hydrogen (H₂), is suitable. Hydrogen may bespecially preferred since it shows a low absorption of radiation in theEUV range. The driver gas inside cavity 22 is used as a source ofelectrons to start a discharge between anode and cathode. In someapplications a separate driver vapor may be dispensed with when anyresidual background pressure of working vapor, or any other vaporpresent, will prove to be sufficient inside cathode cavity 22.

The cathode cavity 22 surrounds a working vapor source 60, which ejectsa working vapor in the anode-cathode gap in a region around central axisA. The working vapor is chosen for its spectral emission as a plasma.The present embodiment uses lithium (Li) for its very strong emissionline at approximately 13.5 nm. Xenon (Xe) may also be used, which has abroad emission spectrum in the XUV (and EUV) region of theelectromagnetic radiation spectrum. Another option is to employ tin (Sn)instead of lithium. The Li source 60 shown comprises a heater 61 below acontainer 62 containing solid lithium to be converted into a liquid andvapor by heater 61. Vaporized Li reaches the anode-cathode gap through aLaval nozzle 63.

A trigger electrode 50 is inserted in cathode cavity 22. Electrode 50 isconnected to appropriate electrical circuitry (not shown in FIG. 3) forapplying a voltage pulse to the electrode to start the dischargedescribed with respect to FIGS. 2b and 2 c. In the state shown in FIG.2b, the radiation source is close to auto-triggering. A voltage pulseapplied to trigger electrode 50 causes a disturbance of the electricalfield within cathode cavity 22, which will cause triggering of thehollow cathode and the formation of a breakdown channel and subsequentlya discharge between cathode 20 and anode 10. Trigger electrode 50comprises a ring surrounding axis A within cathode cavity 22 and isconnected to outside electronics.

Radiation emitted from a collapsed plasma will pass through an opening11 in the anode 10 into a vacuum chamber 70 that is evacuated throughopening 71 in a wall of the vacuum chamber. Plasma and debris particlesmay also escape through opening 11. A flywheel shutter 80 is present toblock these particles when no XUV radiation pulse is emitted forpreventing them to reach any optical elements in the radiation path ofthe XUV radiation to the projection system PL.

Embodiment 3

FIG. 4 depicts a third embodiment of the invention, which is a variationto the second embodiment and further shields the aperture region ofcathode 20 from plasma collapse at central axis A. Both anode 10 andcathode 20 have a “hat-like” structure. Annular cathode cavity 22 andaperture 21 are located at the bottom side of the hat. A plasma createdby a discharge at aperture 21 will compress upwards and “around thecorner” towards central axis A. Further, the positions of anode 10 andcathode 20 have been interchanged. Cathode 20 is located on the outsideof the configuration and comprises aperture 23 for passing XUV radiationto vacuum chamber 70. Trigger electrode 50 in this embodiment takes theform of a needle electrode inserted in cathode cavity 22.

However, the density of the working vapor, also Li vapor in the presentembodiment, may be too low at annular aperture 21 of cathode 20 forcreating a discharge and a plasma. In embodiment 3, the radiation sourceis configured so as to yield a sufficiently high pressure of the drivervapor, Ar in the present embodiment, within the anode-cathode gap in theregion at the annular aperture 21 for creating a discharge in the drivergas. The resulting plasma of the driver gas will start to compresstowards central axis A and at some point encounter a sufficiently highpressure of the working vapor to create a plasma of the working vapor,which will then further compress until stagnation on central axis A. Theplasma of the driver vapor may even first have to go “around the corner”to reach a sufficiently high pressure of the working vapor.

Embodiment 4

FIG. 5 depicts an electrical circuit 100 of a radiation source accordingto a fourth embodiment of the invention and may be employed with theembodiments disclosed above. Electrical circuit 100 and a triggerelectrode 50 may also be employed with a discharge plasma radiationsource having a central hollow cathode, but are favorably employed withthe annular hollow cathode concept.

In circuit 100 an AC voltage is rectified by rectifier 101 and appliedto first capacitor bank C1 in a closed state of electronicallycontrollable switch 110. Switch 110 may be put in a closed state byapplying an appropriate pulse. First capacitor C1 is charged to anominal voltage during a time interval that is (slightly) shorter than atime interval between subsequent XUV radiation pulses.

When first capacitor C1 is charged, a second electronically controllableswitch 120 is closed to charge second capacitor bank C2 through atransformer TV1 within a few microseconds to a working voltage in therange of 3-5 kV with a stored energy in the range of 5-10 J. Secondcapacitor bank C2 is connected to anode and cathode of the radiationsource. Second capacitor bank C2 corresponds to capacitor bank 40 shownin FIGS. 3 and 4.

Capacitor bank C2 will decharge when a discharge occurs between anode 10and cathode 20. The discharge may be triggered as described with regardto the second embodiment using a trigger electrode 50 inserted in thecathode cavity 22. A voltage pulse may be supplied to trigger electrode50 via a second transformer TV2.

Embodiment 5

FIG. 6 schematically shows another embodiment of the invention, whichincorporates a structure to pass lithium, or another appropriatematerial such as tin (Sn), to the anode-cathode gap by capillary action.Embodiment 5 corresponds to the previous embodiments save as describedbelow. The structure may take the form of a tube 90 having an internaldiameter of, for instance, 3 mm, one end of tube 90 being inserted in abath 62 of lithium that is heated by heater 61 to such a temperature (inthe order of 300° C. or above) so as to make the lithium liquid. Theliquid lithium is drawn into the internal passage of the tube andtransported to the other end by capillary action. At this end thelithium is heated by another heater 91 (to approximately 700° C.) tohave the lithium leave the tube into the anode-cathode gap. The lithiumwill leave to tube according to some angular profile that generally ispeaked along axis A. Depending on the dimensions of the exit opening ofthe tube, the lithium may leave the tube as a jet having an angularprofile that is more sharply peaked than a thermal angular (cosine)distribution.

FIG. 7 shows a porous rod 95 that may replace tube 90 in FIG. 6 fortransporting lithium by capillary action to the anode-cathode gap. Achamber 96 is provided at the end of rod 95 near the anode-cathode gapto collect the lithium transported through the porous rod out of lithiumbath 62. An opening 97 is provided in the chamber for the lithium toescape to the anode-cathode gap. Depending on the dimensions of theopening, the temperature of the lithium in the chamber (heated by heater91 to a temperature in the order of 700° C.) and the pressure of thelithium that builds up in the chamber, the lithium will leave opening 97as a jet having some angular profile that may be sharply peaked alongaxis A.

FIG. 6 shows further that the electrical insulator 30 between anode andcathode is positioned at some distance from the region where the lithiumis introduced in the anode-cathode gap. Lithium may chemically reactwith a ceramic isolator provided in between anode and cathode so as toerode it and/or make it electrically conductive, which both are to beavoided. To this end provisions are taken to prevent that lithium vaporin the anode-cathode gap will reach insulator 30. Measures areincorporated to condense lithium vapor before it may reach insulator 30and the condensed lithium may be collected and transferred to lithiumbath 62.

FIG. 6 shows that the distance between the hot region of theanode-cathode gap where the plasma is formed and insulator 30 provides asufficiently long and narrow path 150 having such a temperature that atsome point along that path a ‘freezing point’ for vaporized lithium isprovided. The path is sufficiently long and narrow to provide cooling oflithium vapor upon collision with the wall of the path. Additionally,the path may have a curved stretch 151 to yield an increased number ofcollisions between that wall and the vapor as is shown in the right-handside of FIG. 6. Further, cooling elements 152 may be provided along thepath as shown in the left-hand side. Path 150 presents in the embodimentshown upper and lower walls 155, 156 that circumscribe axis A, upperwall 155 being electrically connected to anode 10 and lower wall 156being electrically connected to cathode 20.

For the lithium to condense, the temperature at the freezing point alongthe path should be at approximately 300° C. or below. Having atemperature at approximately 300° C. will keep the lithium liquid sothat it can be easily collected and returned to the lithium bath. FromFIG. 6 it is obvious how this is achieved by having the liquid flow bygravity down the wall of the path towards lithium bath 62.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

What is claimed is:
 1. A radiation source comprising: a first electrode having an aperture substantially centered around a central axis of said radiation source for passing electromagnetic radiation from said radiation source; and a second electrode spaced apart from said first electrode to form a gap therebetween, said gap defining a discharge region, said gap being supplied with a working vapor, wherein one of said first electrode and said second electrode includes a hollow cavity in communication with said discharge region through an aperture that has a substantially annular configuration around the central axis of said radiation source, said hollow cavity is supplied with a driver gas.
 2. A radiation source according to claim 1, wherein said first electrode is an anode and said second electrode is a cathode.
 3. A radiation source according to claim 1, wherein said first electrode is a cathode and said second electrode is an anode.
 4. A radiation source according to claim 1, wherein said cavity has a substantially annular configuration around the central axis of the radiation source.
 5. A radiation source according to claim 1, wherein said first electrode and said second electrode are connected to different electrical potentials such that a plasma discharge is initiated in said driver gas inside said hollow cavity, followed by a compression of the plasma of the driver gas towards said central axis of said radiation source in which said plasma of the driver gas encounters the working vapor to create a plasma in the working vapor, the plasma in the working vapor emitting said electromagnetic radiation.
 6. A radiation source according to claim 1, wherein said driver gas comprises at least one selected from the group comprising helium, neon, argon and hydrogen.
 7. A radiation source according to claim 1, further comprising: a shutter disposed in the vicinity the aperture of the first electrode, wherein said shutter is adapted to substantially block particles formed in said discharge region.
 8. A radiation source according to claim 7, wherein said shutter includes a flywheel.
 9. A radiation source according to claim 7, wherein said shutter is adapted to open the aperture of the first electrode to let radiation pass through and to close the aperture of the first electrode to substantially block said particles.
 10. A radiation source according to claim 1, wherein said working vapor is supplied in a region proximate said central axis of said gap between said first electrode and said second electrode.
 11. A radiation source according to claim 10, wherein said working vapor is supplied along said central axis.
 12. A radiation source according to claim 1, wherein said working vapor comprises xenon.
 13. A radiation source according to claim 1, wherein said working vapor comprises at least one selected from the group comprising lithium vapor and tin vapor.
 14. A radiation source according to claim 13, further comprising: a reservoir adapted to contain a material comprising at least one of lithium and tin; a heater arranged to heat at least a portion of said reservoir so as to create a vapor from said material; and a fluid passageway in communication with said reservoir to allow said vapor to enter said gap between said first electrode and said second electrode.
 15. A radiation source according to claim 13, further comprising: a reservoir adapted to contain a material comprising at least one of lithium and tin; a first heater arranged to heat at least a portion of said reservoir so as to create a liquid from said material; and a fluid passageway in communication with said reservoir, said liquid created from said material is drawn inside said fluid passageway by capillary action.
 16. A radiation source according to claim 15, further comprising: a second heater in contact with a portion of said fluid passageway, wherein said second heater is arranged to heat at least a portion of said liquid drawn inside said fluid passageway so as to create a vapor from said liquid and to allow said vapor to enter said gap between said first electrode and said second electrode by capillary action.
 17. A radiation source according to claim 15, wherein said fluid passageway includes a tubular section.
 18. A radiation source according to claim 15, wherein said fluid passageway includes a porous rod.
 19. A radiation source according to claim 18, wherein said porous rod is terminated at one of its ends with a chamber, said chamber being adapted to collect at least one of the lithium vapor and the tin vapor.
 20. A radiation source according to claim 19, wherein said chamber includes an opening through which at least one of said lithium vapor and tin vapor escapes to said gap between said first electrode and said second electrode.
 21. A radiation source according to claim 1, further comprising: an electrical insulator disposed between said first electrode and said second electrode; and a canal leading to a said gap between said first electrode and said second electrode, wherein said electrical insulator is disposed inside said canal away from said working vapor.
 22. A radiation source according to claim 21, wherein said canal defines a path along which said working vapor condenses to form a liquid material.
 23. A radiation source according to claim 22, wherein a temperature along said path is less than or equal to 300° C.
 24. A radiation source according to claim 21, wherein said canal is inclined relative to a wall of a reservoir in said radiation source such that the liquid material is collected by gravity in said reservoir.
 25. A radiation source according to claim 21, wherein said canal comprises a curved portion.
 26. A radiation source according to claim 1, further comprising a trigger electrode, wherein at least a portion of said electrode is disposed within said hollow cavity.
 27. A radiation source according to claim 26, further comprising an electrical circuit constructed and arranged to apply a voltage pulse to said trigger electrode.
 28. A radiation source according to claim 27, wherein said electrical circuit comprises a transformer having primary and secondary windings, said primary windings being in electrical communication with a voltage source to supply said voltage pulse and said secondary windings being in electrical communication with one of said first electrode and second electrode and said trigger electrode.
 29. A radiation source according to claim 1, wherein said radiation source is adapted to generate a beam of radiation having a wavelength between about 5 nm and about 20 nm.
 30. A lithographic projection apparatus comprising: a radiation system adapted to provide a projection beam of radiation; a support structure adapted to support a patterning structure to pattern the projection beam according to a desired pattern; a substrate table adapted to hold a substrate; and a projection system disposed between said support structure and said substrate table, said projection system being configured to project the patterned beam onto a target portion of the substrate, wherein said radiation system comprises: a first electrode having an aperture centered around a central axis of said radiation source; and a second electrode spaced apart from said first electrode to form a gap therebetween, said gap defining a discharge region, said gap being supplied with a working vapor, wherein one of said first electrode and said second electrode includes a hollow cavity in communication with said discharge region through an aperture that has a substantially annular configuration around the central axis of said radiation source, said hollow cavity is supplied with a driver gas.
 31. A radiation source comprising: a plasma chamber adapted to house a plasma, said plasma chamber having an aperture through which a radiation emitted by said plasma passes; a shutter disposed in a vicinity of said aperture, wherein said shutter is adapted to substantially block particles formed in said plasma.
 32. A radiation source according to claim 31, wherein said shutter includes a flywheel.
 33. A radiation source according to claim 31, wherein said shutter is adapted to open the aperture to let said radiation emitted by said plasma to pass through and to close the aperture to substantially block said particles formed in said plasma.
 34. A radiation source according to claim 31, wherein a first wall of said chamber forms a first electrode and a second wall of said chamber forms a second electrode.
 35. A radiation source comprising: a source of material including at least one of lithium and tin; an electrode disposed in the vicinity of said source, wherein said electrode is configured to induce formation of a plasma in said at least one of the lithium and the tin, said plasma emitting a radiation having a wavelength in extreme ultraviolet range of wavelengths.
 36. A radiation source according to claim 35, wherein said extreme range of wavelengths is between about 5 nm and about 20 nm.
 37. A radiation source according to claim 35, further comprising: a heater arranged to heat at least a portion of said source of material so as to create a vapor from said material; and a fluid passageway in communication with said source of material to allow said vapor from said material to enter a region in said radiation source in which said plasma takes place.
 38. A radiation source according to claim 37, wherein said fluid passageway includes a porous rod.
 39. A radiation source according to claim 38, wherein said porous rod is terminated at one of its ends with a chamber, said chamber being adapted to collect a vapor from said material.
 40. A radiation source according to claim 39, wherein said chamber includes an opening through which said vapor from said material escapes to said region in said radiation source in which said plasma takes place. 